Loudspeaker

ABSTRACT

Conventional analogue loudspeakers have a limited dynamic range as compared to the available dynamic range of digital recordings. Digital recordings use up to 24 bits and this implies a dynamic range of 141 dB. Digital loudspeakers, involving 2?N  single bit device (with N=24, this number is 1.7×10 7 ) have been proposed. Complexity and poor performance associated with the interaction effects between the different devices have discouraged widespread use of such systems, along with the inability, of most loudspeakers to reproduce realistic absolute levels of sound (up to say 120 dB at 1 m without distortion), so such digital loudspeakers cannot take full advantage of the 24-bit fidelity. This invention provides a loudspeaker with improved dynamic range over prior art systems.

[0001] This invention relates to loudspeakers and in particular toloudspeakers with improved dynamic range as compared to existingloudspeakers.

[0002] Conventional (or single input) loudspeaker systems can be definedas systems in which the master drive signal may be passed to a pluralityof drivers, but for which, at any particular frequency the relationshipbetween the signals passed to each driver is fixed. A driver in thiscontext could mean an electro-magnetic induction coil (as used inconventional loudspeakers) or a piezo-electric pad or any other devicethat can cause a panel-form loudspeaker or a loudspeaker cone to move.

[0003]FIG. 1 shows a conventional loudspeaker system comprising threedrivers/loudspeakers 1, 2, 3. A master signal 4 is split by filters 5, 6and 7 (high pass filter, band pass filter and low pass filterrespectively) into three frequency ranges, treble 5 a which goes tospeaker 1, mid-range 6 a which goes to speaker 2 and bass 7 a which goesto speaker 3. This represents a multiple speaker system in which thereis a frequency split of the main master drive signal 4. The relationshipbetween each of the drivers 1, 2 and 3 is fixed and is not dependent onthe level of the master signal.

[0004] Conventional analogue loudspeakers have a limited dynamic rangeas compared to the available dynamic range of the latest digitalrecordings (for example 24 bit or DSD). Digital recordings use up to 24bits and this implies a dynamic range of 141 dB. Digital loudspeakers,involving 2^(N) single bit devices (with N=24, this number is 1.7×10⁷)have been proposed—see WO96/31086. However, these suffer from obviouscomplexity and poor performance associated with the interaction effectsbetween the different devices, which have discouraged widespread use ofsuch systems. A further problem is the inability of most loudspeakers toreproduce realistic absolute levels of sound (up to say 120 dB at 1 mwithout distortion), so such digital loudspeakers cannot take fulladvantage of the 24-bit fidelity.

[0005] A conventional loudspeaker system, such as that shown in FIG. 1,will suffer distortion and other detrimental effects if the dynamicrange supplied to any of the drivers/loudspeakers 1, 2 or 3 exceeds muchmore than 100 dB. Note, although conventional speakers can beconstructed to have a dynamic range of approaching 120 dB they are veryexpensive. More usually the dynamic range of a conventional speaker isin the region of 100 dB.

[0006] It is therefore an object of the present invention to provide aloudspeaker system, which overcomes or at least mitigates theabove-mentioned problems with prior art systems.

[0007] Accordingly this invention provides a “multiple input loudspeakersystem” (as herein defined) comprising one or more loudspeakers and aplurality of analogue drives arranged in use to drive the one or moreloudspeakers wherein, in use, the one or more loudspeakers are input adrive signal having a time varying signal level and, at any particulartime, the signal level measured at the input to the loudspeaker systemdetermines the operational state of each of the drivers.

[0008] A “multiple input loudspeaker” may be made up from a plurality ofconventional analogue loudspeakers or alternatively from a panel-formloudspeaker, sometimes referred to as a flat panel loudspeaker or amulti-mode radiator, having a plurality of analogue drivers.

[0009] A “multiple input loudspeaker” is not a conventional multiplechannel loudspeaker system (used for example in surround sound or stereosound systems) although it could be applied to such a multiple channelsystem.

[0010] “Multiple input loudspeaker” systems in contrast may be definedas systems in which a master drive signal is devoted into a plurality ofsignals which are applied to a plurality of drivers but for which at anyparticular frequency, the relationship between the signals passed toeach driver depends on the level of the master signal. This distinctionis illustrated in FIGS. 1 and 2.

[0011]FIG. 1 as described above represents a conventional loudspeakersystem. FIG. 2 shows a multiple input loudspeaker system covered by theinvention comprising a number of drivers 10 a, 10 b, 10 c, 10 d . . . 10n, which receive their input from a master signal 8. Note, this mastersignal could be the same as master signal 4 in FIG. 1 or it couldrepresent one of the channels 5 a, 6 a or 7 a or any other aspect of anaudio system.

[0012] Each master signal 8, see FIG. 2, is a time varying data steam,and it is this varying amplitude level that determines the signal sentto each drive 10 a . . . 10 n.

[0013] By choosing suitable factors in the calculation of the drivesignals for each driver it is possible to make sure that no driver isoverloaded and each will operate within its linear dynamic range withlow distortion.

[0014] The plurality of analogue drivers can be connected toconventional speakers or more conveniently the plurality of drivers candrive a single panel-form loudspeaker.

[0015] Panel-form loudspeaker technology is able to take advantage ofdigital fidelity because it is able to inherently produce very highabsolute levels of sound. By using a panel-form loudspeaker combinedwith a plurality of analogue drivers or exciters it is possible toovercome the problems of complexity, interaction effects and loudnesswhich limit the benefits of existing solutions. Prior art devices havesuggested the use of more than one driver for a single loudspeaker, butnone of them have recognised the need to control how these driversinteract to obtain the benefits of the present invention.

[0016] Irrespective of the choice of loudspeaker (i.e. panel-form orconventional) there are a number of alternative algorithms by which theanalogue drivers can be controlled.

[0017] In a first algorithm, an oversampling method is used. The signalto each driver is determined at each digital data point usingINT{(x+k)/n) for the kth driver, 0≦k<n, where x is the basic signallevel expressed as a signed integer, n is the number of drivers andINT{} implies the lowest integer part of. This algorithm is shown inFIG. 3 for a full level sine wave with 16 drivers. This algorithm iscomplex, but overcomes most problems associated with the use ofconventional loudspeakers for digital recordings, because all driversare always activated and all drivers use substantially the same waveformas shown in FIG. 3.

[0018] Alternatively, in a second algorithm, a first driver is activatedand driven until the signal level reaches a first predetermined level, asecond driver is activated when the signal level reaches the firstpredetermined level; and subsequent drivers are activated as the signallevel reaches subsequent respective predetermined levels, whereby allactivated drivers share load equally at all activated levels.

[0019] Alternatively, in a third algorithm, a first driver is drivenuntil the signal level reaches a first predetermined level, wherein asecond driver is activated as the signal level reaches the firstpredetermined level; wherein subsequent drivers are activated as thesignal level reaches subsequent respective predetermined levels; wherebyeach newly activated driver takes the load required and all otheractivated drivers are saturated. This algorithm is shown in FIG. 4 for afull level sine wave with 16 drivers.

[0020] For Algorithm 1 all drivers are activated at all signal levels.Algorithms 2 and 3 have the advantage that at low signal levels only asingle driver is activated, thus potentially giving higher quality soundat such levels than would be the case with algorithm 1. Algorithm 3 hasthe advantage of only having signal gradient discontinuities at thechange over levels—thus reducing unwanted transient switching problems.

[0021] Preferably for algorithms 2 and 3, an exponential or othersmoothing function is applied to the control signal for each newlyactivated driver such that the addition of a new driver to all the otheractivated drivers is achieved in a continuous manner.

[0022] Algorithms 2 and 3 can be considered as producing drive signalswith effective time-varying gain. However, rapid changes in the gainassociated with each driver can cause undesirable non-linear distortioneffects and therefore a still further way of controlling the drivers isto control the rate at which the gain to each driver changes so that itis changed in a smooth fashion. Therefore, preferably, a smoothingfunction is first applied to the master drive signal at the input to theloudspeaker. The smoothed drive signal can then be used to calculate thenumber of operational drivers required.

[0023] A window, such as a sliding boxcar, can be employed successfullyin this “smoothing” role. Whereby, the gain applied to each driver isbased on the weighted average signal measured as the mean across anumber of samples which encompass points both in the first and the past,relative to the current time sample of the master drive signal. Thus,for any time t, the gain is calculated from a weighted mean signalbetween the times t−mΔt and t+nΔt, where Δt is the time betweenindividual signal samples and m and n are integers. These integers maybe equal or may be chosen to favour either the past or future portionsof the signal. The total duration of the window (m+n) Δt effectivelycontrols the rate at which the gain to each driver changes. Thissmoothing box-car function is illustrated in FIG. 5 wherein an initiallyrapidly changing signal in FIG. 5a is smoothed by the action of the boxcar function into the smooth signal of FIG. 5b.

[0024] Since the loudest elements of music signals tend to occur at thelowest frequencies, the width of the window can be chosen to properlyproduce the necessary low frequency signals whilst avoiding rapidchanges in gain to each loudspeaker.

[0025] Preferably, at very low levels only one driver is activated andat very high levels all drivers are activated, and the sum of all thedriver outputs equals the required signal outputs at all times.

[0026] At low frequencies the acoustic pressures produced by the actionof each active driver will tend to add in a linear fashion. In order toensure that the combined output from all drivers is correct a controlsignal can conveniently be applied to the linear time signal to maintainthe sum of the linear time output equal to the required signal output.

[0027] In contrast, at high frequencies the acoustic pressures producedby the action of each active driver will add in a power manner.Therefore in order to ensure that the combined power output is correct acontrol signal can conveniently be applied to a suitable squared timesignal such that the sum of the acoustic power output is equal to thedesired power output. This is beneficial at the higher frequencies wheredrivers tend to act independently of one another.

[0028] Preferably, the controller operates in both linear and powersignals, such that at low frequencies the controller maintains thelinear sum, whilst at high frequencies the controller maintains thepower sum. This arrangement covers a wide frequency range.

[0029] Embodiments of the loudspeaker system according to the presentinvention will now be described with reference to the accompanyingdrawings in which:

[0030]FIG. 1 illustrates a conventional multi-channel loudspeakersystem;

[0031]FIG. 2 illustrates a multiple-input loudspeaker system accordingto the invention;

[0032]FIG. 3 illustrates an algorithm (=algorithm 1) to controloperation of a loudspeaker according to the present invention.

[0033]FIG. 4 illustrates an algorithm (=algorithm 3) to controloperation of a loudspeaker according to the present invention;

[0034]FIG. 5 illustrates the sliding boxcar averaging process todetermine the controlling master amplitude, according to the presentinvention;

[0035]FIG. 6 is a block diagram of a panel-form loudspeaker inaccordance with the present invention;

[0036]FIG. 7 illustrates one example of a radiator and drivers for apanel-form loudspeaker in accordance with the present invention;

[0037]FIG. 8 illustrates another example of a radiator and drivers for apanel form loudspeaker in accordance with the present invention;

[0038]FIGS. 9 and 10 illustrate a suitable smoothing function (for usewith algorithm 3) to apply to each driver such that new drivers arebrought in smoothly; and

[0039]FIG. 11 illustrates an alternative structure of panel-formloudspeaker in accordance with the present invention.

[0040] Note: throughout all the Figures like numerals are used to denotelike features.

[0041]FIG. 6 shows one example of a panel-form loudspeaker according tothe present invention. A signal, for example from an amplifier (notshown) is input to a control processor 11. The output of the controlprocessor 11 modifies the operation of one or more drivers 10, which areattached to a radiator panel 12 and when operated excite a multi moderesponse of the panel (Note: this arrangement is equivalent to the oneshown in FIG. 2, i.e. there are a number of drivers 10 a . . . 10 n. Theonly difference is the speaker technology used, conventional speakers inFIG. 2 and a panel form loudspeaker in FIG. 6).

[0042] The panel is provided with a plurality of drivers, which arearranged across the panel. The arrangement of multiple drivers aims toexcite all modes of the panel and to avoid interactions with each other.This can be achieved using a spiral starting just off-centre or anirregular pattern, both producing driver locations spread throughout thepanel. Alternatively, drivers may be arranged in a more regular manner,either concentrated at the centre or spread across the panel. This isstill effective because the panels themselves tend to be slightlyirregular when manufactured. FIGS. 7 and 8 illustrate two arrangementsof multiple drivers, although others are possible.

[0043] In use, the panel-form loudspeaker of the present invention isoperated by the control processor comparing the input or base signalwith a set of known criteria and then controlling the operation of thedrivers in response to this. For example, an oversampling method can beused. The signal to each driver 10 is determined at each digital datapoint using INT{(x+k)/n} for the kth driver, 0≦k<n, where x is the basicsignal level expressed as a signed integer, n is the number of driversand INT{} implies the lowest integer part of. This algorithm is shown inFIG. 3 for a full level sine wave with 16 drivers. This example has theadvantage that all drivers use substantially the same waveform as shownin FIG. 3.

[0044] In a second example, one driver 10 a is always driven and forlevels of the base signal, which fall within its dynamic range, this isthe only driver activated. When the level of the signal goes above this,another driver 10 b is switched on such that both now share the loadequality (i.e. at changeover the signal to the original driver is halvedand this same half signal is sent to the second driver). When the levelexceeds that which can be accommodated by two drivers, a further driver10 c will be switched on such that all three now share the load equallyand so on until all drivers are in use. This particular embodiment cansuffer from a problem of significant transients and distortionsoccurring at changeover, but it has the advantage of being particularlyeasy to implement.

[0045] In a third example, one driver 10 a is always driven and forlevels of the base signal which fall within its dynamic range this isthe only driver activated. When the level of the signal goes above this,another driver 10 b is switch on to add to the first driver 10 a, butthe first driver 10 a is left saturated such that at the changeover thesecond driver 10 b is at its minimum level. When the level exceeds thatwhich can be accommodated by two drivers, a further driver 10 c will beswitched on and so on until all drivers are in use. This algorithm isshown in FIG. 4 for a full level sine wave with 16 drivers. This thirdexample has the advantage of only having signal gradient discontinuitiesat the change over levels—thus reducing unwanted transient switchingproblems.

[0046] The type of input signal used by the control processor to controlthe drivers is dependent on the frequency. At low frequencies, e.g.below 300 Hz, use of linear signals is preferred because the whole panelmoves in monophase and at higher frequencies, e.g. greater than 500 Hz,power signals are preferred because multi-modal resonances are excitedin the radiator as described in EP0541646. In the crossover regionbetween 300 Hz and 500 Hz, the signals will be partially linear andpartially power signal. The invention applies to any size ofloudspeaker. However, at the low frequency end there may need to be aminimum size to obtain the benefits of the present invention.

[0047] A further improvement is to apply a smoothing function to thecontrol signal applied to each newly activated driver, so that the newdriver is brought in in a continuous manner, rather than a step change.An example of a suitable smoothing function is a tanh function as shownin FIG. 9 for four drivers. As a new driver is added, the signalscombine smoothly until the total required level is reached, asillustrated by FIG. 10.

[0048] In a fourth example (see FIG. 5) the gain associated with thesignal for each driver is smoothed in the time domain using a moving,short duration averaging algorithm. This smoothed amplitude signal isused as the master control to decide the gain of each driver. Inessence, each driver receives the original waveform but at a levelcontrolled by the smoothed level of the original waveform.

[0049] This example is illustrated in FIG. 5. An input signal isdepicted in FIG. 5 as having a rapidly changing level. Controlling thedrivers based on this drive signal could cause non-linear distortioneffects and so a boxcar smoothing function is applied to the signal inorder to produce the smooth signal depicted in FIG. 5b. This smoothsignal can now be used to determine the number of drivers to be used. Inthis case the aforementioned algorithm 3 is used and subsequent driversare activated as the signal level reaches subsequent respectivepredetermined levels (see FIG. 5c). An exponential smoothing functionhas not been applied in this instance.

[0050] Another feature of the invention is to drive a single panel-formloudspeaker with a number of drivers, possibly identical. The driversare activated to simulate a conventional loudspeaker i.e. more aredriven for low frequency signals than for high frequency signals, inorder to be able to handle the required power. Signals to the driverscould be the same, but a digital filter in front of each driver wouldcontrol the frequency range over which each is driven. A computer couldmanipulate the filter cut off and gain. This would allow the system tobe tailored for different room settings and for the drivers to beswitched on and off according to the absolute power levels required.

[0051] In order to further improve the loudspeaker performance, thepanel may be constructed in a tapered form as shown in FIG. 11. Thepanel has a sandwich structure, so that it operates in a region aboveacoustic coincidence for the greater part of the frequency range. Hence,the coincidence frequency varies according to panel position. Two skins14, 15 are positioned either side of a cellular core 16. The core may bea honeycomb or other cellular structure.

1. A “multiple input loudspeaker system” (as herein defined) comprisingone or more loudspeakers and a plurality of analogue drivers arranged inuse to drive the one or more loudspeakers wherein, in use, the one ormore loudspeakers are input a drive signal having a time varying signallevel and, at any particular time, the signal level measured at theinput to the loudspeaker system determines the operational state of eachof the drivers.
 2. A multiple input loudspeaker system as claimed inclaim 1 wherein the plurality of analogue drivers drive a plurality ofconventional loudspeakers.
 3. A multiple input loudspeaker system asclaimed in claim 1 wherein the plurality of analogue drivers drive apanel-form loudspeaker.
 4. A multiple input loudspeaker system accordingto any of claims 1 to 3, wherein all drivers are driven and the signallevel input to each driver is the lowest integer part of the basicsignal level expressed as a signed integer plus the number of the drivein question, over the total number of drivers.
 5. A multiple inputloudspeaker system according to any preceding claim, wherein a firstdriver is activated and driven until the signal level reaches a firstpredetermined level; wherein a second driver is activated when thesignal level reaches the first predetermined level; and whereinsubsequent drivers are activated as the signal level reaches subsequentrespective predetermined levels; whereby all activated drivers shareload equality at all activated levels.
 6. A multiple input loudspeakersystem according to any of claims 1 to 3, wherein a first driver isdriven until the signal level reaches a first predetermined level,wherein a second driver is activated as the signal level reaches thefirst predetermined level; wherein subsequent drivers are activated asthe signal level reaches subsequent respective predetermined levels;whereby each newly activated driver takes the load required and allother activated drivers are saturated.
 7. A multiple input loudspeakersystem according to either claim 5 or 6 wherein the addition of a newdriver is achieved in a continuous manner by applying an exponential orother smoothing function to the signal sent to the drivers.
 8. Amultiple input loudspeaker system according to any preceding claimwherein at very low levels only one driver is activated and at very highlevels all drivers are activated; and wherein the sum of all the driveroutputs equals the required signal outputs at all times.
 9. A multipleinput loudspeaker system according to any preceding claim wherein acontrol signal is applied to the linear time signal to maintain the sumof the linear time output equal to the required signal output.
 10. Amultiple input loudspeaker system according to any preceding claimwherein a control signal is applied to a suitable squared time signalsuch that the sum of the acoustic power output is equal to the desiredpower output.
 11. A multiple input loudspeaker system according to anypreceding claim wherein the control signal operates in both linear andpower signals, such that at low frequencies the controller maintains thelinear sum whilst at high frequencies the controller maintains the powersum.
 12. A multiple input loudspeaker system as hereinbefore describedwith reference to and/or as illustrated in the accompanying drawings.13. A panel-form loudspeaker system as hereinbefore described withreference to and/or as illustrated in the accompanying drawings.